1) Field of the Invention
The present invention relates to methods and devices for fabricating semiconductor devices and more particularly a method and a device for monitoring the dimensions of submicron photoresist patterns, contact openings, and metal via openings.
2) Description of the Prior Art
In recent years contact openings and metal via openings have become smaller and have reached submicron sizes. The processes to fabricate these structures have become complex and expensive. Moreover, several device parameters must be measured and monitored to ensure that the photolithography processes are run properly and that product devices are correct. These measurements include hole dimensions, step heights, and proximity effects in cells. Hole dimensions are measured to monitor the photo mask quality, exposure times, photoresist quality and other process variables. Step height is the difference in heights of different layers, for example, the height difference between the substrate surface and the top of a field oxide region. It is important to measure step heights since large step heights cause subsequent overlying conductive layers to have "step coverage" problems. These step coverage problems occur where the overlying conductive layer thins or breaks as it crosses over from a thick to a thin underlying layer boundary. Proximity effects are scattered electron beams that adjacent pattern elements receive from other closely spaced pattern elements. As a result of the proximity effects, it is necessary to adjust exposure time for various classes of elements by pattern density.
The monitor patterns and monitor test sites currently used to monitor semiconductor device dimensions have several deficiencies that do not allow optimal measurements to be taken. FIG. 1 shows a top down view of a typical monitor pattern. Hole patterns 7 and line patterns 9 are formed in layers 11 of the substrate surface. The current monitor patterns do not allow accurate measurements of step heights. Also, proximity effects in cells can not be accurately measured on the present monitor test sites because the monitor test sites do not include structures which precisely replicate the variations in pattern spacings in actual devices. Moreover, the current test monitor is difficult to use with automatic measurement equipment because the present monitor sites do not contain targets large enough for the automatic measuring equipment to align to.
In order to obtain some critical dimension measurements, manufacturing operators attempt to make measurements on device cells. Unfortunately, because of the small device size and complex pattern, the operator's manual measurements of device cells lead to errors. The manual measurements are also time consuming. Moreover, it is not possible to make some manual measurements, e.g., hole dimensions and different height steps, on actual devices because the devices are have been reduced to sub micron sizes.
There is a need to develop a new monitor pattern which will overcome these problems. The monitor pattern should allow easy step height and hole dimension measurements in addition to including areas for measuring proximity effects. Automeasurement tools should be able to make measurements on the new monitor patterns.